Page 60 - Industrial Wastewater Treatment, Recycling and Reuse

Industrial Wastewater Treatment, Recycling, and Reuse: An Overview 43

1. Cost and energy-intensive oxygen transfer is avoided. No large aerators
are required.
2. Space–time yields (bioreactor performances) are far better than in the
activated-sludge process because of the limitations imposed by oxygen
transfer.
3. Since oxygen is not used, aerosol formation is avoided apart from strip-
ping out the volatile components.
4. There is effective removal of heavy metals through reductive precipita-
tion (as heavy-metal sulfides) rather than oxidative precipitation.
5. Energy is utilized in the form of biogas.
6. The anaerobic process represents true waste disposal with very little
sludge generated. Nearly 95% of the organic contamination is converted
into a combustible gas.
A popular mode involves a two-stage operation: stage I for acidogenic
microorganisms and stage II for mainly methanogenic microorganisms.
The methanogenic operation is usually implemented in the form of an
upflow anaerobic sludge blanket (UASB) or fixed-bed reactor. Although
a single-stage operation requires lower initial investment than a two-stage
system, today, the trend in the anaerobic treatment of highly contaminated
wastewater is toward a two-stage process design. There have been a number
of approaches for retention and recycling of the biomass that serves as catalyst
for different steps involved in the anaerobic process. The most important
methods for decoupling the residence times for liquid substrate and biomass
in anaerobic wastewater treatment include either internal retention or
external separation and recycling.
Biomass retention (internal)
1. Sedimentation by pellet formation
– UASB
2. Filtration
– Membrane anaerobic reaction system
– Rotor-fermenter
3. Immobilization by adsorption
– Fixed-bed reactor
– Fixed-bed loop reactor
– Anaerobic film reactor
– Fluidized-bed reactor
– Hybrid concepts (UASB/fixed bed)
– Stirred-tank reactor with suspended carriers
4. Immobilization by inclusion or covalent bonding